Solar cell front contact doping

ABSTRACT

A method of doping solar cell front contact can improve the efficiency of CdTe-based or other kinds of solar cells.

CLAIM OF PRIORITY

This application claims priority to U.S. Provisional Patent ApplicationNo. 61/224,941, filed on Jul. 13, 2009, which is incorporated byreference in its entirety.

TECHNICAL FIELD

This invention relates to a solar cell with a doped front contact.

BACKGROUND

Photovoltaic devices can use transparent thin films that are alsoconductors of electrical charge. The conductive thin films can includetransparent conductive layers that contain a transparent conductiveoxide (TCO), such as tin oxide or zinc oxide. The TCO can allow light topass through a semiconductor window layer to the active light absorbingmaterial and also serve as an ohmic contact to transport photogeneratedcharge carriers away from the light absorbing material.

DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic of a photovoltaic device having multiplesemiconductor layers and a metal back contact.

FIG. 2 is a schematic of a photovoltaic device having multiplesemiconductor layers and a metal back contact.

FIG. 3 is a process flow chart of making a doped sputter target.

FIG. 4 is a schematic showing the sputtering deposition process of afront contact layer.

FIG. 5 is a process flow chart of deposition and processing a frontcontact layer including a wash/rinse step.

FIG. 6 is a schematic illustrating the wash/rinse doping process of asubstrate.

FIG. 7 is a schematic illustrating the wash/rinse doping process of asubstrate.

FIG. 8 is a schematic illustrating the wash/rinse doping process of asubstrate.

FIG. 9 is a schematic illustrating the wash/rinse doping process of asubstrate.

FIG. 10 is a schematic illustrating the substrate and transparentconductive oxide layer after an annealing process.

DETAILED DESCRIPTION

During the fabrication of photovoltaic devices, layers of semiconductormaterial can be deposited on a substrate including a front contact andan absorber layer. The front contact can include a semiconductor windowlayer allowing the penetration of solar radiation to the absorber layer,where the optical power is converted into electrical power. Some tophotovoltaic devices can use transparent thin films that are alsoconductors of electrical charge. The front contact can also includetransparent conductive layers that contain a transparent conductiveoxide (TCO), such as cadmium stannate. The TCO can allow light to passthrough a semiconductor window layer to the active light absorbingmaterial and also serve as an ohmic contact to transport photo-generatedcharge carriers away from the light absorbing material. In certainembodiments, the front contact can also include the layers positionedbetween the TCO and the absorber layer.

For thin film solar cells, the transparent conductive oxide (TCO)material can influence device performance. TCO layers with highelectrical conductivity can be desirable. For a TCO layer made of zincoxide or tin oxide, its thickness can be increased to lower the sheetresistance. In practice, the thick TCO layer can result in costincrease, peeling and adhesion problems, and manufacturing difficulties.Methods of making doped TCO layer are developed to reduce the sheetresistance of solar cell front contacts without increasing theirthickness.

In one aspect, a photovoltaic device can include a substrate, a frontcontact layer adjacent to the substrate, wherein the front contact layeris doped by a dopant, and a semiconductor absorber layer adjacent to thedoped front contact layer, wherein the semiconductor absorber layer caninclude cadmium telluride. The front contact layer can include atransparent conductive oxide layer adjacent to the substrate and abuffer layer adjacent to the transparent conductive oxide layer. Thefront contact layer can further include a semiconductor window layeradjacent to the buffer layer. The transparent conductive oxide layer caninclude a zinc oxide. The transparent conductive oxide layer can includea tin oxide. The dopant can include an N-type dopant. The dopant caninclude at least one element selected from the group consisting ofaluminum, indium, boron, copper, chlorine, gallium, fluorine, andmagnesium. The substrate can include glass. The buffer layer can includeat least one component selected from the group consisting of zinctelluride, cadmium zinc telluride, and cadmium sulfide. The window layercan include an M_(1-x)G_(x)O_(y) semiconductor, the M is selected fromthe group consisting of zinc and tin, and the G is selected from thegroup consisting of aluminum, silicon, and zirconium, or at least onecomponent selected from the group consisting of cadmium sulfide, zinctelluride, cadmium zinc sulfide, zinc oxide, zinc sulfide, zincmagnesium oxide, cadmium magnesium sulfide, zinc aluminum oxide, zincsilicon oxide, zinc zirconium oxide, tin aluminum oxide, tin siliconoxide, tin zirconium oxide, and cadmium oxide.

In another aspect, a method of manufacturing a photovoltaic device caninclude depositing a transparent conductive oxide layer adjacent to asubstrate. After the deposition, the transparent conductive oxide layerhas a surface. The method can include exposing the surface of thetransparent conductive oxide layer to a dopant, wherein a dopant layercan be left on the surface, depositing a window layer adjacent to thedopant layer, incorporating the dopant into the window layer, anddepositing an absorber layer adjacent to the window layer. The absorberlayer can include cadmium telluride. Depositing the window layer caninclude a sputtering process. Exposing the surface of the transparentconductive oxide layer can include washing the surface with a salt ofthe dopant. The salt can include a boric-acid salt. The dopant caninclude an N-type dopant. The dopant can include at least one elementselected from the group consisting of aluminum, indium, boron, copper,chlorine, gallium, fluorine, and magnesium.

The window layer can include at least one component selected from thegroup consisting of cadmium sulfide, zinc telluride, cadmium zincsulfide, zinc oxide, zinc sulfide, zinc magnesium oxide, cadmiummagnesium sulfide, and cadmium oxide. The window layer can include anM_(1-x)G_(x)O_(y) semiconductor, the M can be selected from the groupconsisting of zinc and tin, and the G can be selected from the groupconsisting of aluminum, silicon, and zirconium. Depositing the windowlayer can include a vapor transport deposition process. Depositing thewindow layer can include depositing a cadmium sulfide window layeradjacent to the dopant layer and depositing a zinc-containing layer onthe cadmium sulfide window layer. The sputtering process can includesputtering from a ceramic target in an inert atmosphere or reactivesputtering from a metal target. The method can further include anannealing step.

In another aspect, a method of manufacturing a photovoltaic device caninclude depositing a transparent conductive oxide layer adjacent to asubstrate, depositing a window layer adjacent to the transparentconductive oxide layer by sputtering from a doped target, and depositingan absorber layer adjacent to the window layer. The doped target can bedoped by a dopant. The absorber layer can include cadmium telluride. Themethod can include depositing a precursor layer adjacent to thetransparent conductive oxide layer by sputtering from a doped target,depositing a buffer layer adjacent to a precursor layer, and annealingthe precursor layer and the buffer layer to form a doped buffer layer.

A photovoltaic device can include a transparent conductive oxide layeradjacent to a substrate and layers of semiconductor material. The layersof semiconductor material can include a bi-layer, which may include ann-type semiconductor window layer, and a p-type semiconductor absorberlayer. Photons can free electron-hole pairs upon making contact with then-type window layer, sending electrons to the n side and holes to the pside. Electrons can flow back to the p side via an external currentpath. The resulting electron flow provides current which, combined withthe resulting voltage from the electric field, creates power. The resultis the conversion of photon energy into electric power.

Referring to FIG. 1, a photovoltaic device 100 can include a dopedtransparent conductive oxide layer 120 deposited adjacent to a substrate110. Transparent conductive oxide layer 120 can be deposited onsubstrate 110 by sputtering, chemical vapor deposition, or any othersuitable deposition method. Substrate 110 can include a glass, such assoda-lime glass. Transparent conductive oxide layer 120 can include anysuitable transparent conductive oxide material, including a cadmiumstannate, an indium-doped cadmium oxide, or a tin-doped indium oxide. Asemiconductor bi-layer 130 can be formed or deposited adjacent totransparent conductive oxide layer 120 which can be annealed.Semiconductor bi-layer 130 can include semiconductor window layer 131and semiconductor absorber layer 132. Semiconductor window layer 131 ofsemiconductor bi-layer 130 can be deposited adjacent to transparentconductive oxide layer 120. Semiconductor window layer 131 can includeany suitable window material, such as cadmium sulfide, and can bedeposited by any suitable deposition method, such as sputtering or vaportransport deposition. Semiconductor absorber layer 132 can be depositedadjacent to semiconductor window layer 131. Semiconductor absorber layer132 can be deposited on semiconductor window layer 131. Semiconductorabsorber layer 132 can be any suitable absorber material, such ascadmium telluride, and can be deposited by any suitable method, such assputtering or vapor transport deposition. Back contact 140 can bedeposited adjacent to semiconductor absorber layer 132. Back contact 140can be deposited adjacent to semiconductor bi-layer 130. A back support150 can be positioned adjacent to back contact 140. A photovoltaicdevice can have a Cadmium Sulfide (CdS) layer as a semiconductor windowlayer and a Cadmium Telluride (CdTe) layer as a semiconductor absorberlayer.

An alternative configuration of photovoltaic devices consistsessentially of three semiconductor materials and will be referred to asa “p-i-n device”. The “i” stands for “intrinsic” and refers to asemiconductor material that in equilibrium has a relatively low numberof charge carriers or either charge type or in which the net number ofcarriers, where “net” would be the absolute value of the concentrationof p-type charge carriers minus the concentration on n-type carriers,can be less than about 5×10¹⁴ cm⁻³, wherein 2×10¹⁵ cm⁻³ is the upperlimit for a p-i-n device. Typically the primary function of the “i”layer is to absorb optical photons and convert them to electron-holepairs. The photogenerated electrons and holes move within the “i” layeras driven by drift and diffusion until they either “recombine” with eachother within the i-layer or at the p-i interface or at the i-n interfaceor until they are collected by the n and p layers, respectively.

Referring to FIG. 2, a photovoltaic device 200 can include a transparentconductive oxide layer 220 deposited adjacent to a substrate 210.Transparent conductive oxide layer 220 can be deposited on substrate 210by sputtering, chemical vapor deposition, or any other suitabledeposition method.

A sputtering target can be manufactured by ingot metallurgy. Asputtering target can include one or more components of a layer or filmto be deposited or otherwise formed on a surface, such as a substrate.For example, a sputtering target can include one or more components of aTCO layer to be deposited on a substrate, such as zinc for a zinc oxideTCO layer, tin for a tin oxide TCO layer, or a dopant such as an N-typedopant or P-type dopant, such as aluminum, indium, boron, chlorine,copper, gallium, or fluorine. The sputtering target can include one ormore components of a window layer to be deposited, such as cadmium for acadmium sulfide window layer, zinc for a zinc oxide window layer, or adopant such as an N-type dopant or P-type dopant, such as aluminum,indium, boron, chlorine, copper, gallium, fluorine, or magnesium. Thecomponents can be present in the target in stoichiometrically properamounts. A sputtering target can be manufactured as a single piece inany suitable shape. A sputtering target can be a tube. A sputteringtarget can be manufactured by casting a metallic material into anysuitable shape, such as a tube. A sputtering target can be manufacturedfrom more than one piece. A sputtering target can be manufactured frommore than one piece of metal, for example, a piece of zinc for a zincoxide TCO and a piece of dopant material, such as aluminum. Thecomponents can be formed in any suitable shape, such as sleeves, and canbe joined or connected in any suitable manner or configuration. Forexample, a piece of zinc and a piece of aluminum can be welded togetherto form the sputtering target. One sleeve can be positioned withinanother sleeve.

A sputtering target can be manufactured by powder metallurgy. Asputtering target can be formed by consolidating metallic powder to formthe target. The metallic powder can be consolidated in any suitableprocess (e.g., pressing such as isostatic pressing) and in any suitableshape. The consolidating can occur at any suitable temperature. Asputtering target can be formed from metallic powder including more thanone metal powder. More than one metallic powder can be present instoichiometrically proper amounts.

A sputter target can be manufactured by positioning wire includingtarget material adjacent to a base. For example wire including targetmaterial can be wrapped around a base tube. The wire can includemultiple metals present in stoichiometrically proper amounts. The basetube can be formed from a material that will not be sputtered. The wirecan be pressed (e.g., by isostatic pressing).

A sputter target can be manufactured by spraying a target material ontoa base. Metallic target material can be sprayed by any suitable sprayingprocess, including thermal spraying and plasma spraying. The metallictarget material can include multiple metals, present instoichiometrically proper amounts. The base onto which the metallictarget material is sprayed can be a tube.

Substrate 210 can include a glass, such as soda-lime glass. Transparentconductive oxide layer 220 can include any suitable transparentconductive oxide material, including a cadmium stannate, an indium-dopedcadmium oxide, or a tin-doped indium oxide. A semiconductor tri-layerstructure 230 can be formed or deposited adjacent to annealedtransparent conductive oxide layer 220. Semiconductor tri-layerstructure 230 can include first semiconductor layer 231, semiconductorinterfacial layer 232, and second semiconductor layer 233. Firstsemiconductor window layer 231 of semiconductor tri-layer structure 230can be deposited adjacent to annealed transparent conductive oxide layer220. First semiconductor window layer 231 can include any suitablewindow material, such as cadmium sulfide, and can be formed by anysuitable deposition method, such as sputtering or vapor transportdeposition. Semiconductor interfacial layer 232 can be depositedadjacent to first semiconductor layer 231. Semiconductor interfaciallayer 232 can be any suitable semiconductor material, such as zinctelluride, and can be deposited by any suitable method, such assputtering or vapor transport deposition. Second semiconductor layer 233can be deposited on semiconductor interfacial layer 232. Secondsemiconductor layer 233 can be any suitable semiconductor material, suchas cadmium telluride, and can be deposited by any suitable method, suchas sputtering or vapor transport deposition. Back contact 240 can bedeposited adjacent to second semiconductor layer 233. Back contact 240can be deposited adjacent to semiconductor tri-layer structure 230. Aback support 250 can be positioned adjacent to back contact 240.

TCO layers with high optical transmission, high electrical conductivityand good light scattering properties are always desirable. A TCO layercan have a sheet resistance about 5 ohm per square or even lower. For aTCO layer made of pure zinc oxide or tin oxide, its thickness sheetresistance can be lowered (for example to about 5 ohms per square) byincreasing layer thickness. In practice, the thick TCO layer can resultin cost increase. Cracks can also appear in thick TCO films, leading topeeling and adhesion problems. Furthermore, very thick TCO films cancreate supplementary difficulties while patterning the TCO during theproduction step of series connection for module production.

A TCO layer can be doped to reduce the sheet resistance of solar cellfront contacts without increasing their thickness. Methods of makingdoped TCO layer can include a sputter process from a doped target.

A doped window layer can be deposited using a similar process. Incertain embodiments, a method of manufacturing a photovoltaic devicewith doped window layer can include the steps of depositing atransparent conductive oxide layer adjacent to a substrate, depositing awindow layer adjacent to the transparent conductive oxide layer bysputtering from a doped target, and depositing an absorber layeradjacent to the window layer, wherein the absorber layer can includecadmium telluride.

The deposition of the window layer can further include the steps ofdepositing a cadmium sulfide window layer adjacent to the dopant layerand depositing a zinc-containing layer on the cadmium sulfide windowlayer.

In certain embodiments, the sputtering process of doped TCO or dopedwindow layer can include sputtering from a ceramic target in an inertatmosphere or reactive sputtering from a metal target.

Referring to FIG. 3, making a doped sputter target can include the stepsof preparing raw material oxide powders with at least one dopant,canning the powders, hot isostatic pressing the powders with at leastone dopant, machining to final form, final clean, and inspection. Makinga doped sputter target can further include annealing or any othersuitable metallurgy technique or other treatment. The doped sputtertarget can include an oxide such as tin oxide or tin oxide with at leastone dopant such as boron, sodium, fluorine, or aluminum.

Referring to FIG. 4, front contact sputter system 300 can includechamber 310 and radio-frequency source and matching circuit 360.Substrate 370 can be mounted on plate 380 or positioned in any othersuitable manner. Grounded fixture 330 can hold doped sputter target 340facing down. The gas in chamber 310 is taken from inlet 320 with sourcesof different gas. The gas in chamber 310 can include argon. Duringsputtering process, particles 350 can be deposited from target 340 tosubstrate 370. The sputtering process can be a reactive sputteringprocess. The gas in chamber 310 can further include dopant gascontaining boron, sodium, fluorine, or aluminum. System 300 can includeoutlet 390 to exhaust gas. In other embodiments, the sputtering processcan be a DC sputtering, magnetron sputter deposition, or ion assisteddeposition.

Referring to FIG. 5, deposition and processing a front contact layer caninclude the steps of substrate wash/rinse, sputter deposition, or anyother suitable post-process step. During the step of substratewash/rinse, salts can be added to process the substrate. The salts caninclude borax. The deposition and processing can further include anyother suitable chemical bath step. The deposition and processing canfurther include forming a transparent conductive oxide precursor layeron the surface of the substrate for further processing.

Methods of making doped TCO layer can also include a substrate washprocess. It can include the steps of washing the substrate with a dopantand depositing a doped transparent conductive oxide layer adjacent to asubstrate, wherein the transparent conductive oxide layer becomes dopedby the dopant. Washing a surface can include spraying, dipping, rollercoating, misting, or spin coating a fluid onto the surface. The dopantcan be a component of a wash fluid which can optionally includeadditional components. The substrate can be washed with a salt of thedopant, such as borax.

Referring to FIG. 6 through FIG. 9, a wash/rinse doping process of asubstrate is illustrated. As shown in FIG. 6, substrate 410 can bewashed with dopant source 430. Mask 420 can optionally be used to coverregions that do not need to be washed. Mask 420 can be photoresist orany suitable mask material. Salts can be added to dopant source 430, forexample borax. As shown in FIG. 7, the wash/rinse process can leave alayer 440 including the dopant adjacent to the substrate. As shown inFIG. 8, transparent conductive oxide precursor layer 450 can bedeposited adjacent to layer 440 including the dopant. Transparentconductive oxide precursor layer 450 can include zinc or tin. Anadditional annealing process converts the precursor layer 450 into atransparent conductive oxide layer that can include a zinc oxide or atin oxide. Dopant atoms can diffuse into substitutional positions in thecrystal lattice, resulting in desired changes in the electricalproperties of the semiconductor material. As shown in FIG. 9,transparent conductive oxide precursor layer 450 can become doped by thedopant from layer 440. Transparent conductive oxide layer 450 can alsobecome doped by the dopant left on substrate 410 by dopant source 430.Referring to FIG. 10, after annealing, substrate 410 is in contact withtransparent conductive oxide layer 460 which has a doped region 451including an N-type dopant, sodium, boron, fluorine.

The process can include a heat treatment or any suitable drive-intreatment after wash. The process can also include an additionaldiffusion doping process with impurity ions in gaseous form. The methodsof making doped TCO layer can also include an additional step ofannealing the substrate after the doped transparent conductive oxidelayer is deposited.

A doped window layer can be deposited using similar process. In certainembodiments, a method of manufacturing a photovoltaic device with adoped front contact can include the steps of depositing a transparentconductive oxide layer adjacent to a substrate, exposing the surface ofthe transparent conductive oxide layer to a dopant, wherein a dopantlayer can be left on the surface, depositing a window layer adjacent tothe dopant layer, incorporating the dopant into the window layer, anddepositing an absorber layer adjacent to the window layer, wherein theabsorber layer comprises cadmium telluride.

In certain embodiments, a method of manufacturing a photovoltaic devicewith a doped front contact can include the steps of depositing atransparent conductive oxide layer adjacent to a substrate, depositing aprecursor layer adjacent to the transparent conductive oxide layer bysputtering from a doped target, depositing a buffer layer adjacent to aprecursor layer, annealing the precursor layer and the buffer layer toform a doped buffer layer, and depositing an absorber layer adjacent tothe doped buffer layer, wherein the absorber layer comprises cadmiumtelluride.

In certain embodiments, the front contact layers can include the layersbetween the TCO layer and the semiconductor absorber layer. The frontcontact layers can include cadmium sulfide. The front contact layers canalso have tin oxide/cadmium sulfide stack structure or any suitablemodified structure based on that. The front contact layers can bedeposited by reactive sputtering, vapor transport deposition, or anyother suitable deposition method. The front contact layers can also bedoped by a suitable dopant. The dopant can include an N-type dopant orP-type dopant, such as aluminum, indium, boron, chlorine, copper,gallium, or fluorine.

In certain embodiments, the method of manufacturing a photovoltaicdevice can include washing a surface of a substrate with a dopant,depositing a semiconductor window layer on the washed surface of thesubstrate and incorporating the dopant into the semiconductor windowlayer, and depositing a semiconductor absorber layer adjacent to thesemiconductor window layer. The substrate can include a glass sheet anda transparent conductive oxide stack. The semiconductor window layer caninclude cadmium sulfide, zinc telluride, cadmium zinc sulfide, zincoxide, zinc sulfide, or zinc magnesium oxide. Depositing thesemiconductor window layer can include sputtering, vapor transportdeposition, or any other suitable deposition method. Washing thesubstrate can further include a step of exposing the surface of thesubstrate with a salt of the dopant. The dopant can include an N-typedopant or P-type dopant, such as aluminum, indium, boron, chlorine,copper, gallium, or fluorine. The semiconductor absorber layer caninclude cadmium telluride. The transparent conductive oxide stack caninclude a tin oxide or zinc oxide. The resulted dopant concentration inthe semiconductor window layer can be in the range from about 10¹³ cm⁻³to about 10²⁰cm⁻³ or from about 10¹⁴ cm⁻³ to about 10¹⁹cm⁻³.

In certain embodiments, the method of manufacturing a photovoltaicdevice can include depositing a semiconductor window layer adjacent to asubstrate, wherein the semiconductor window layer is doped by a dopantand depositing a semiconductor absorber layer adjacent to the dopedsemiconductor window layer. The substrate can include a glass sheet anda transparent conductive oxide stack. The semiconductor window layer caninclude cadmium sulfide, zinc telluride, cadmium zinc sulfide, zincoxide, zinc sulfide, or zinc magnesium oxide. Depositing thesemiconductor window layer can include sputtering from a doped target,vapor transport deposition, or any other suitable deposition method. Thedopant can include an N-type dopant or P-type dopant, such as aluminum,indium, boron, chlorine, copper, gallium, or fluorine. The semiconductorabsorber layer can include cadmium telluride. The transparent conductiveoxide stack can include a tin oxide. The transparent conductive oxidestack can include a zinc oxide. The resulted dopant concentration in thesemiconductor window layer can be in the range from about 10¹³ cm⁻³ toabout 10²⁰cm⁻³ or from about 10¹⁴ cm⁻³ to about 10¹⁹cm⁻³.

A number of embodiments of the invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention. Itshould also be understood that the appended drawings are not necessarilyto scale, presenting a somewhat simplified representation of variouspreferred features illustrative of the basic principles of theinvention.

1-9. (canceled)
 10. A method of manufacturing a photovoltaic device comprising: depositing a transparent conductive oxide layer adjacent to a substrate, wherein, after the deposition, the transparent conductive oxide layer has a surface; exposing the surface of the transparent conductive oxide layer to a dopant, wherein a dopant layer can be left on the surface; depositing a window layer adjacent to the dopant layer; incorporating the dopant into the window layer; and depositing an absorber layer adjacent to the window layer, wherein the absorber layer comprises cadmium telluride.
 11. The method of claim 10, wherein depositing the window layer comprises a sputtering process.
 12. The method of claim 10, wherein exposing the surface of the transparent conductive oxide layer comprises washing the surface with a salt of the dopant.
 13. The method of claim 12, wherein the salt includes a boric-acid salt.
 14. The method of claim 10, wherein the dopant comprises an N-type dopant.
 15. The method of claim 10, wherein the dopant comprises at least one element selected from the group consisting of aluminum, indium, boron, copper, chlorine, gallium, fluorine, and magnesium.
 16. The method of claim 10, wherein the window layer comprises at least one component selected from the group consisting of cadmium sulfide, zinc telluride, cadmium zinc sulfide, zinc oxide, zinc sulfide, zinc magnesium oxide, cadmium magnesium sulfide, and cadmium oxide.
 17. The method of claim 10, wherein the window layer comprises an Mi-xGxOy semiconductor, the M is selected from the group consisting of zinc and tin, and the G is selected from the group consisting of aluminum, silicon, and zirconium.
 18. The method of claim 10, wherein depositing the window layer comprises a vapor transport deposition process.
 19. The method of claim 10, wherein depositing the window layer comprises: depositing a cadmium sulfide window layer adjacent to the dopant layer; and depositing a zinc-containing layer on the cadmium sulfide window layer
 20. The method of claim 11, wherein the sputtering process comprises sputtering from a ceramic target in an inert atmosphere or reactive sputtering from a metal target.
 21. The method of claim 10, further comprising an annealing step.
 22. A method of manufacturing a photovoltaic device comprising: depositing a transparent conductive oxide layer adjacent to a substrate; depositing a window layer adjacent to the transparent conductive oxide layer by sputtering from a doped target, wherein the doped target is doped by a dopant; and depositing an absorber layer adjacent to the window layer, wherein the absorber layer comprises cadmium telluride.
 23. The method of claim 22, further comprising: depositing a precursor layer adjacent to the transparent conductive oxide layer by sputtering from a doped target, wherein the doped target is doped by a dopant; depositing a buffer layer adjacent to a precursor layer; and annealing the precursor layer and the buffer layer to form a doped buffer layer. 